Abrasive grinding machines typically employ a wide endless abrasive belt that is movably supported in tension between at least two aligned rollers one of which is driven. In a two-roller grinding head, one roller is disposed above the other, the lower roller being driven and serving as the contact roller with workpieces moved thereunder by a conveyor belt. In three-roller grinding heads, two lower rollers spaced identically from the conveyor belt are used with an upper roller. A platen is disposed between the lower rollers in backing support to the abrasive belt to effect the desired abrading function.
In either case, the endless belt is driven around the rollers longitudinally either against or with movement of the workpiece, depending on the surfacing function to be accomplished. In addition, it is now well recognized that controlled, lateral movement or tracking of the endless abrasive belt back and forth across the rollers can be beneficial to the overall process to achieve more uniform wear of the abrasive belt as well as to more effectively deal with the limited degree of inherent lateral belt movement.
Consequently, it is conventional for abrasive grinding machines to include some type of tracking means for causing the wide abrasive belt to move back and forth within predetermined outer limits; i.e., the belt is caused to move laterally in one direction to a predetermined point at one end of the rollers, at which point belt movement is reversed to a predetermined point at the opposite end of the rollers. The cycle repeats on a uniform periodic basis.
One type of tracking means consists of a cradle in which the upper roller is mounted so that it not only rotates about its own longitudinal axis, but also pivotally swings back and forth about a pivot axis perpendicular to the rotational axis and passing through its midpoint. This permits the upper roller to pivotally swing either direction from a position in which it is parallel with the lower roller or rollers into a nonparallel or skew position. With the upper roller in nonparallel position, tension on the belt is non-uniform and it moves laterally over the belt in a direction corresponding to the direction and degree of skew.
Back and forth swinging movement of the cradle is accomplished with a pair of pneumatic actuators that are mounted on opposite sides of the cradle and are alternately actuated as a function of belt position. Position of the belt is sensed by a detector that conventionally takes the form of a light source and photocell that are disposed on opposite sides of one flight of the belt, so that one edge traverses the light beam as the belt moves laterally in one direction and leaves the beam as it moves in the opposite direction. The pneumatic actuators are alternately actuated as the light beam is broken and then reestablished.
Several problems arise with belt movement detectors due to the nature of abrasive grinding machines. First, it is difficult if not impossible to utilize a detector that senses movement by actual contact with the belt because the belt moves at a significant linear velocity and is abrasive. The light beam/photocell approach is a satisfactory solution to this problem, but creates other problems of its own. For example, it requires two components (i.e., the light source and the photocell detector) that must be mounted in opposition on opposite sides of the belt in order to permit the belt edge to traverse the light beam. This inherently creates alignment problems between the two components due to machine vibration or other movement. Further, while one component may be protectively mounted inside the belt (i.e., between the belt flights), the other component must be disposed externally of the belt in a position which is far more vulnerable to misalignment and/or damage. Further, the external component is directly exposed to wood dust that is carried and thrown by the external abrasive face of the belt during the grinding or surfacing operation. Although most abrasive grinding machines include dust collection systems, none is totally effective in dust removal and the external detecting component must be positioned in this dusty environment. This obviously has an adverse effect on its ability to properly generate or sense the light beam, and it is possible for the detector to generate a spurious signal due to improper sensing of a dust buildup.